1. Field of the Invention
The present invention relates to a method for producing high purity tetrasodium salt of ethylenediaminetetraacetic acid(EDTA-4Na). More particularly, it relates to a method for producing high purity and uniform crystals of EDTA-4Na from an aqueous solution of crude EDTA-4Na by crystallization using a mixed solvent.
2. Description of the Related Art
Tetrasodium salt of ethylenediaminetetraacetic acid (`Tetrasodium EDTA` or EDTA-4Na) is a white crystalline powder of no taste and no odor. It has a good solubility in water (103 g/100 ml water), although it is rarely soluble in organic solvents such as alcohols or ethers, etc.
EDTA-4Na and its derivatives, which act as sequestering agents or chelating agents, have a wide range of applications in dyeing, photographs, cosmetics, cleaning agents and synthetic resin industries, and the like. They are employed to avoid problems associated with metal ions in water or aqueous solutions by forming chelate complexes of the metal ions.
It may be prepared by reacting ethylene diamine, NaCN and formalin in the presence of an alkali at 60-150.degree. C. as shown in Reaction Scheme 1 below to yield an aqueous solution of crude EDTA-4Na. When by-product ammonia is removed under reduced pressure, a solution of crude EDTA-4Na of about 40-50% purity is obtained. The solution still contains a large amount of impurities such as unreacted reactants and by-products (JP 57-45425A, JP 56-3930 A, U.S. Pat. No. 2,860,164). ##STR1##
These impurities limit the applications of the product EDTA-4Na and it is required to recover purified EDTA-4Na by various purification method in order to use it in foodstuffs or cosmetic industries, etc.
Crystallization based on solubility difference over temperature range is not efficient for purifying EDTA-4Na because the difference is not significant as shown in FIG. 1.
Four typical methods for recovering crystals of EDTA-4Na have been known.
In one method so called `total drying`, the aqueous solution of crude EDTA-4Na of about 40% purity is dried by using spray dryer. This method yields only low purity EDTA-4Na product and cannot obtain high purity product.
In a second method, the aqueous solution of crude EDTA-4Na (about 40% purity) was adjusted to lower than pH 2 with HCI to precipitate an insoluble acid form of EDTA (U.S. Pat. No. 2,860,164). The acid form of EDTA obtained is washed with a large amount of water to completely remove by-product salts and four(4) equivalents of NaOH is added thereto to yield high purity EDTA-4Na (Reaction Scheme 2). ##STR2##
Although this method produces high purity EDTA-4Na, it still has several problems in association with the steps of crude EDTA-4Na.fwdarw.EDTA.fwdarw.EDTA-4Na. Thus, the reaction of crude EDTA-4Na and HCl gives a reaction waste containing a large amount of by-product salts and the addition of NaOH to Na-free EDTA to obtain EDTA-4Na increases production cost.
In a third method, 50% NaOH is used to salt out EDTA-4Na crystals (JP 62-61022B). This method produces an excessive NaOH-containing waste which causes environmental pollution, making it commercially disadvantageous.
In a fourth method, a single organic solvent, in particular methanol is used to precipitate high purity EDTA-4Na crystals. This method still has several problems in that the yield is not satisfactory and that the crystals of EDTA-4Na obtained are distributed in a wide range of particle diameters and the microparticles having a diameter less than about 30 .mu.m occupy a large portion, requiring a long filtration time for recovery, that eventually contributes to the decrease in productivity (Comparative Examples 2-9).
Therefore, a new effective method for recovering high purity EDTA-4Na is needed.